The present invention relates to a plasticating control apparatus for controlling plasticating conditions to stabilize the temperature of a material to be plasticated, and molten and kneading states thereof in an injection machine.
Unlike an extrusion machine wherein a screw in a barrel (i.e., a heating cylinder) is continuously rotated, a screw in an in-line screw type injection machine periodically repeats rotation and stopping in accordance with a plasticating measuring operation and an injection action. During the plasticating process, since the screw is moved backward while a material, such as a molten resin, is being fed from a hopper, the effective length of the screw constantly changes. The temperature of the molten resin and the molten and kneading states thereof become nonuniform due to the basic operations described above, resulting in degradation of quality of finished products. In order to prevent this, conventionally, plasticating control is performed by controlling a backward movement speed of the screw during the plasticating process in the injection machine. In order to control the backward movement speed of the screw, various control apparatuses have been conventionally proposed wherein a rotational speed of the screw is adjusted to control the action of the molten resin fed in front of the screw, and simultaneously or independently, a back pressure of a hydraulic cylinder is adjusted to control the backward resistive force of the screw.
However, the backward resistive force is a sum of the back pressure of the hydraulic cylinder, a frictional force between the screw and the heating cylinder, a frictional force between the molten resin in the screw channel and the heating cylinder, a packing resistance in the hydraulic cylinder, and a sliding resistive force of a hydraulic motor unit. These forces interfere with the backward movement of the screw. The molten resin pressure must be equal to or higher than the backward resistive force. Even if the back pressure becomes zero, the molten resin pressure cannot be zero but has a given value. Therefore, when only the back pressure is controlled, plastication cannot be performed at a molten resin pressure lower than the back pressure. For example, when plasticating control is improper under the condition where the molten resin pressure is near zero, the backward movement of the screw is pulsated or the backward speed is locally decreased. In a worst case, depending upon the type of resin, the screw is stopped.
Furthermore, since the screw back pressure adjustment indirectly controls the backward speed of the screw, the adjustment itself is very difficult. Even if the back pressure of the screw is controlled, a time lag occurs in a hydraulic piping system due to changes in volumes caused by expansion/contraction of a working fluid and changes in pressures of the hydraulic piping system. The molten resin pressure cannot be immediately controlled, resulting in inconvenience. Furthermore, it is very difficult to control the backward resistive force depending upon the molding conditions and the type of resin supplied to a general-purpose screw.